It is well known to tone electrostatic latent images contained on photoconductive or dielectric surfaces by application thereto of electroscopic marking particles. It is also known to have electroscopic marking particles in dry form which can be fixed by pressure onto the photoconductor which may comprise a sheet of paper coated with photoconductive zinc oxide in a resinous binder or which electroscopic marking particles can be transferred from the photoconductor onto plain paper and affixed thereto by pressure. Electroscopic marking particles which can be fixed by pressure are generally referred to as pressure fixing dry toner.
Pressure fixing dry toner composition of encapsulated and non-encapsulated type are also well known. Encapsulated toners are disclosed for instance in U.S. Pat. Nos. 3,080,250, 3,080,251, 3,080,318, 3,893,932 and 3,893,933. Pressure fixing toners of non-encapsulated type are disclosed for instance in U.S. Pat. Nos. 3,788,994, 3,804,764, 3,873,325, 3,903,320 and 3,925,219.
Encapsulated dry toners of the prior art methods disclosed have inherent limitations with regards particle size control, quantity and type of colouring matter which can be encapsulated, capsular shell thickness and inertness. In each of the prior art disclosures the fixing pressures disclosed are very high, being generally within the range 200-500 pounds/lineal inch. In the past it was considered that pressure fixing toners were of necessity formulated to require high fixing pressure in order that such materials should possess suitable physical characteristics for normal handling purposes.
In our co-pending U.S. patent application Ser. No. 958,009 dated Nov. 6, 1978 (based on Australian Patent Application No. PD 2374/77 dated Nov. 10, 1977), entitled "Microcapsular Electroscopic Marking Particles" and in co-pending International Application No. PCT/AU81/00179 dated Dec. 3, 1981 (based on Australian Patent Application No. PE6894/80 dated Dec. 11, 1980), entitled "Dry Toner for Electrostatography" there are disclosed electroscopic marking particles that is dry toner material in the form of microcapsules of controlled size wherein high intensity colouring matter can be encapsulated within an inert capsular shell and wherein such capsular shell allows fixing by relatively low pressure yet permits easy handling of the dry toner material. Such electroscopic marking particles are prepared in the form of microcapsules following in general the encapsulation teachings of H. Ruus as disclosed in U.S. Pat. No. 3,429,827 and G. E. Maalouf as disclosed in U.S. Pat. No. 4,000,087, each of which is incorporated herein by reference. The technique disclosed therein is generally referred to as interfacial polycondensation, in which process a non-aqueous phase containing one reacting material is emulsified in an aqueous phase containing a second reacting material. Reaction is arranged to occur under constant agitation to produce microdroplets of the non-aqueous phase encapsulated in a shell comprising the reaction product formed at the phase interface, such shell preferably comprising a substantially impervious polyamide or other polymeric compound.
The microcapsular electroscopic marking particles in accordance with our co-pending applications comprise coloured encapsulated pressure fixable substance contained within a rupturable shell and a residue layer over such shell.
The substance to be encapsulated comprises in essence an ink, a first reactive substance and a carrier for said ink and said first reactive substance.
The ink in accordance with out co-pending applications comprises colouring matter which may be organic or inorganic pigment, magnetite or ferrite or other magnetizable substances, dyes which may be present in particulate state or in dissolved state or in absorbed or adsorbed state associated with the pigment or magnetizable substance, binder material for said colouring matter such as mineral and vegetable oils, natural and synthetic resins, bituminous substances, rubber or other elastomers, waxes, plasticizers, aliphatic or aromatic hydrocarbon solvents, anti-oxidants, viscosity modifying agents, metallic soaps, alkyl aryl compounds, phosphatides such as lecithin, bitumens or asphalts, sulphur containing compounds such as sulphosuccinates and such like substances.
The carrier medium for said ink may comprise a solvent or a plasticizer such as for instance dibutyl phthalate or the like substance having dispersed therein said ink and having dissolved therein said first reactive substance.
To form for instance a polyamide shell in accordance with the interfacial polycondensation process there is first prepared a water phase that is an aqueous solution of an emulsion stabilizer such as polyvinyl alcohol or hydroxy ethyl cellulose or cellulose gum or albumin and the like and then in such solution the aforementioned substance to be encapsulated is dispersed to form an emulsion. Following such emulsification of said substance to be encapsulated the second reactive substance which is an amine containing substance such as diethylene triamine or the like in aqueous solution which may also contain a pH stabilizer such as sodium carbonate or the like is added to such emulsion under agitation and such agitation is continued for some time until a polycondensation product that is polyamide is formed as a shell at the interface between the emulsified droplets or globules of the substance to be encapsulated and said water phase due to reaction between said first and second reactive substances contained in said substance to be encapsulated and said water phase, respectively.
Said water phase may also contain therein in dispersed or dissolved state protective colloids and surface active agents of the anionic, cationic or nonionic type and the like, which substances may adsorb onto or be absorbed by or react in part with the shell.
Upon completion of the interfacial polycondensation process there is formed an aqueous slurry containing therein in suspended state microcapsules and in dissolved or dispersed state at least part of the aforementioned emulsion stabilizer, protective colloid and surface active agent. Such slurry can be spray dried to form discrete capsular particles which contain on the outer shell wall in dry form and adsorbed thereto or absorbed thereby or at least in part reacted therewith a residue layer consisting of said emulsion stabilizer, protective colloid and surface active agent, such dry discrete capsular particles or agglomerates thereof constituting the electroscopic marking particles.
The dry toner powder material in accordance with our co-pending applications is characterised by forming high colour density images as the interfacial polycondensation process lends itself admirably to the inclusion of substantial quantity of colouring matter into the toner produced by such process. In addition to the incorporation of pigments and dyes into the ink which forms part of the encapsulated substance it is also possible to include dyes in dissolved or dispersed state, where such dyes can be dissolved or dispersed in the binder materials for the pigment as previously listed or in the carrier medium such as dibutyl phthalate and the like. Furthermore dyes can be included in the aqueous phase together with the emulsion stabilizer where upon drying such dye is found to be contained on the outside of the capsular shell and bonded thereto by the said emulsion stabilizer. It is also possible to have dyes adsorbed onto the pigment prior to incorporation thereof in said ink in which case such pigment is dyed firstly by milling or mixing in a dye solution and then dried prior to inclusion into the ink with the binder materials.
The pigments and dyes referred to in the foregoing must be so selected that they do not react in ay way with the aforementioned two reactive substances and in particular such dyes must not contain free amine groups which may react with the first reactive substance acid chloride.
In those cases where water dispersible or hydrophilic pigments such as magnetite, ferrite, magnetizable materials and other such like substances are included in the encapsulated substance it is necessary to protect such materials from interaction with the aforementioned first reactive substance and render such materials oil dispersible or oleophilic in order to prevent migration of such particulate matter from the oil phase of the ink in the substance to be encapsulated or from the carrier medium into the aqueous outer phase as such migration precludes effective encapsulation.
Such hydrophilic particulate substances can be protected as well as rendered oleophilic by for instance treating or coating the particle surface with silicones either by the so-called fuming process or by wetting or grinding the particles in a solution of silicones followed by removing the solvent, or by coating the particles by any known method of grinding or dispersing in a solution or hot melt of materials such as natural and synthetic waxes such as polyethylene, oils, synthetic resins such as ketone resins, epichlorhydrin polymers, urethanes, polycarbonates, phthalates, acrylics and styrenes, where some of such materials and in particular the acrylics and styrenes can be applied in substantially monomeric form and subsequently polymerized on the particle surface by known methhods.
While the shell of such microcapsular electroscopic marking particles is not affected by environmental conditions certain emulsion stabilizers such as polyvinyl alcohol which remain on the shell surface upon drying are somewhat affected by moisture and thus it is desirable to render the residual emulsion stabilizer water insoluble which for instance in the case of polyvinyl alcohol can be attained by adding to the aqueous slurry upon completion of the encapsulation process whilst agitating same for some time a water soluble resin such as a melamine resin which reacts with polyvinyl alcohol and renders same water insoluble. Residual emulsion stabilizer or protective colloidsor surface active agents can of course be substantially reduced in quantity by washing the microcapsules with water prior to insolubizing the emulsion stabilizer and if so desired also after insolubilization in order to remove traces of unreacted materials. Alternatively in those instances where the emulsion stabilizer is albumin, it is rendered water insoluble and hydrophobic during the step of spray drying at temperatures above 100.degree. C.
The dry toner powder material consisting of microcapsular electroscopic marking particles in accordance with out co-pending applications can be of polarity suitable for toning of latent images formed by positive or negative electrostatic charges, as desired. Such dry toner powder can be admixed with iron filings for operation in the well known magnetic brush applicators, or with other carrier particles for operation in other toner applicators for instance of the cascade type as is well known. Alternatively in those instances where the encapsulated substance contains magnetite or ferrite or other magnetizable material such dry powder can be employed as a single component toner with a multi magnet roller applicator as is also well known. The dry powder can be applied to tone directly latent images contained on a dielectric or photoconductive surface such as zinc oxide coated paper and fixed thereto or such powder can be employed to tone latent images contained on reusable photoconductors followed by transfer onto plain paper and fixing thereto.
In all instances the toner material can be pressure fixed to the final surfaces by for instance passage through the nip of a pair of pressure rollers which may be heated if so desired where the pressure applied between such rollers can be of lower order than that required for pressure fixing toners of the prior art.
It will be noted that the marking particles produced in accordance with our co-pending applications contain an ink to be encapsulated and a carrier medium for same comprising materials such as polybutene oil, dehydrated castor oil, high boiling range (207.degree.-257.degree. C.) isoparaffinic hydrocarbon, dibutyl phthalate and such like relatively non-volatile liquids which in the course of spray drying at elevated temperatures do not evaporate completely from the capsules. We have found that if the shell of such capsules containing relatively non-volatile liquids is damaged by shear forces or impact during production or in handling such as during spray drying or classification or packaging or during use in a toner applicator, the non-volatile liquid contained therein becomes exposed and upon contact with other capsules acts as adhesive causing cementing together of capsules which then form agglomerates. Such agglomerates formed by capules or toner particles impair the flow properties of the toner in the toner applicator which results in inferior toning efficiency and poor image quality.